This invention relates, in general, to a radar system and, more specifically, to a ladar (laser radar) system having an adjustable, pseudo-noise (PN) coded transmitted signal that is adaptively tuneable to optimize target acquisition.
Laser radar, optical radar and ladar are all names used for radar systems utilizing electromagnetic radiation at optical frequencies. The radiation used by a ladar system is at wavelengths which are 10,000 to 100,000 times shorter than that used by a microwave radar system. Radiation scattered by the target of interest is collected and processed to yield information about the target and range to the target. Early conventional radar and ladar systems observed the intensity of the collected radiation and the time delay from transmission to collection.
Ladar systems may be classified as continuous-wave (CW) or pulsed, as well as focused or collimated. CW ladar systems are generally used when the signal may be integrated over long time periods and/or when the target of interest is nearby. Focusing is mainly performed using CW ladar systems to permit them to make a more sensitive measurement over a smaller span of ranges. In contrast, pulsed ladar systems use much higher peak power levels during the laser pulse than can be maintained with a CW laser, producing higher signal-to-noise ratios for the collected radiation. Pulsed ladar systems are usually chosen for long-range remote sensing and when signal integration over a long time period is impractical.
A ladar system transmits light to a target of interest, the transmitted light interacts with and is changed by the target, and some of this light is reflected/scattered back (returned) to the ladar system where it can be analyzed. For example, the round trip time required for the light to travel to the target of interest and back to the ladar system is used to determine the range to the target.
As known in the art, CW radar systems have been used to acquire a target and measure range to the target. One such system, disclosed in U.S. Pat. No. 5,999,119, issued to Carnes et al. on Dec. 7, 1999, includes changing the phase of a transmitted CW signal by 180 degrees, in accordance with a binary, PN code. A code of N bits, for example, is stored in a re-circulating shift register memory. The bits are read out from memory at a predetermined rate, fs, and fed to a bi-phase modulator along with the CW signal. The phase of the CW signal changes 180 degrees in response to a logic 1 bit and remains unchanged in response to a logic 0 bit. The coded CW signal is transmitted through a transmit antenna. Radar returns from a target are received by a receive antenna. The received CW radar returns have the same code as the transmitted code; however, the received code pattern is time delayed with respect to the transmitted code, by an amount related to the target range. As the target radar returns are correlated with different time delayed replicas of the PN code, target range may be determined.
Another radar system, disclosed in U.S. Pat. No. 6,236,352, issued to Walmsley on May 22, 2001, includes an intermediate frequency (IF) signal used to modulate and demodulate transmitted and received signals, respectively. The IF signal is modulated by a PN coded sequence that has a predetermined number of digital bits. The patent discloses various embodiments, each having a PN coded sequence of different number of bits in length. For example, a PN sequence of one embodiment has 12-bits in length. Another embodiment has 32-bits in length, and yet another has 8-bits in length. The patent does not describe an embodiment having an adaptively tuneable PN sequence, in which the bit length may be adjustable to optimize a received signal to noise ratio (SNR) or acquisition range to a target.
This invention addresses a radar system and a ladar system each including an adjustable, PN coded transmitted signal that is adaptively tuneable to optimize the received SNR, target acquisition range and resolution.
To meet this and other needs, and in view of its purposes, the present invention provides a radar system for acquiring a target including a waveform generator for generating a programmable pseudo-noise (PN) code of a variable code modulation frequency and a variable chip length. Also included are a transmitter for transmitting a continuous wave (CW) radar signal modulated in accordance with the programmable PN code, a receiver adapted to receive a radar return based on the transmitted radar signal, and a processor for processing the received radar return and acquiring the target. The processor includes a first tuning control signal provided to the waveform generator for adjusting the code modulation frequency, and a second tuning control signal provided to the waveform generator for adjusting the chip length, whereby the waveform generator adjusts the code modulation frequency and the chip length, in response to the first and second tuning control signals, to optimize target acquisition.
In an embodiment of the invention, the transmitter includes a laser diode for transmitting a light signal, the light signal modulated in accordance with the programmable PN code, and the receiver includes optics for receiving the light signal.
In one embodiment, the chip length of the programmable PN code varies from a 32 bit sequence to a 1024 bit sequence, and the code modulation frequency varies from 1 MHz to 1024 MHz.
The processor, during a first time period, provides the first tuning control signal to the waveform generator for adjusting the code modulation frequency, and during a second time period, provides the second tuning control signal to the waveform generator for adjusting the chip length, whereby the first time period occurs prior to the second time period.
In another embodiment, the invention includes a ladar system for acquiring an image of a target. The ladar system has a waveform generator for generating a programmable pseudo-noise (PN) code of a variable code modulation frequency and a variable chip length, an optical transmitter for transmitting a continuous wave (CW) laser signal modulated in accordance with the programmable PN code, an optical receiver adapted to receive an image of the target based on the transmitted laser signal, and a processor for processing the received image of the target. The processor includes a first tuning control signal provided to the waveform generator for adjusting the code modulation frequency, and a second tuning control signal provided to the waveform generator for adjusting the chip length, whereby the waveform generator adjusts the code modulation frequency and the chip length, in response to the first and second tuning control signals, to optimize the received image of the target.
The invention also includes a method of dynamically optimizing an acquisition range of a radar system. The method modulates a CW radar signal with a PN code that has an adjustable code frequency of modulation and an adjustable chip length. The method also includes transmitting the modulated radar signal and receiving a return radar signal based on the transmitted radar signal. The method continuously measures a SNR of the received return radar signal and adaptively tunes the adjustable code frequency of modulation, and adaptively tunes the adjustable chip length, based on the continuously measured SNR. In this manner, the acquisition range of the radar system is optimized.
The method further includes determining if the code frequency of modulation is at a minimum value, and increasing the chip length, if the code frequency of modulation is at a minimum value.
The method further determines if the code frequency of modulation is at a maximum value, and decreases the chip length, if the code frequency of modulation is at a maximum value.
In yet another embodiment, the invention includes a ladar system for acquiring an image of a target. The ladar system has a waveform generator for generating a programmable pseudo-noise (PN) code of a variable code modulation frequency and a variable chip length, an optical transmitter for transmitting a continuous wave (CW) laser signal modulated in accordance with the programmable PN code, an optical receiver adapted to receive an image of the target based on the transmitted laser signal, and a processor for processing the received image of the target. The optical receiver includes a silicon detector array for dividing the image into a number of pixels. The received signal from each pixel is processed by autocorrelation of the received PN code to determine a time delay of the signal at a respective pixel. From the time delay, the distance to the corresponding point on the target is calculated. This pixel by pixel distance information is then used to derive the depth dimension of the target allowing improved target recognition. The processor includes a first tuning control signal provided to the waveform generator for adjusting the code modulation frequency, and a second tuning control signal provided to the waveform generator for adjusting the chip length, whereby the waveform generator adjusts the code modulation frequency and the chip length, in response to the first and second tuning control signals, to optimize the received image of the target.
It is understood that the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention.